The technical field of this invention is photo-lithography and, in particular, anti-reflective chemical compounds that attenuate light reflected or scattered by conductor surfaces during device fabrication.
Photo-lithography encompasses a variety of photo-chemical processes used to fabricate integrated circuits. As is generally known, an integrated circuit is typically formed from a small silicon wafer, called a substrate, that has incorporated onto its surface a complete electronic circuit. Each complete circuit is a dense and complex matrix of circuit elements including transistors, capacitors, and wires. As integrated circuits become increasingly dense, the incorporated circuit elements must be made increasingly smaller. Presently, photolithographic processes can produce circuit elements with feature dimensions of approximately one-half a micrometer. Achieving more complex and powerful integrated circuits requires a photolithographic process capable of fabricating circuit elements with design features of about one-quarter of a micrometer. A major obstacle to achieving these sub-micrometer design features is the scattered light that reflects off the substrate during photolithography and interferes with the desired exposure profiles.
Reflected light affects the exposure step of the photolithographic process. During the exposure step, a photosensitive film, called a photo-resist, is exposed to actinic radiation. Typically, the exposure is accomplished by radiating the film with light of a select wavelength through a patterned mask. The exposed resist defines the pattern that is transferred to the substrate, and ultimately incorporated into the integrated circuit. A precise exposure regimen is essential to transfer sub-micrometer design features to the resist. The exposure regimen must control the amount of light absorbed in the photo-resist, and the pattern area affected by the radiation. Both the dose delivered and the pattern transferred must be correct to achieve proper control of sub-micrometer circuit element dimensions.
The accuracy of the exposure regimen is adversely affected by light that reflects off the substrate and travels back through the photo-resist. Such reflected light can travel through previously unexposed photo-resist and essentially "rewrite" the pattern that was transferred. Alternatively, reflected light can travel back through the exposed photo-resist and interfere with the incident radiation to vary the dose of radiation that is delivered to the photo-resist. Depending on the resist thickness, the resulting interference may be constructive or destructive, causing the dose necessary to develop the resist to change as a function of resist thickness. Variations in resist thickness of as little as 10 nm can yield a 10 percent change in absorbed dose intensity. This sensitivity to resist thickness is accentuated at the shorter wavelengths which are necessary to achieve sub-micrometer design features. Collectively, the deleterious effects of reflected light are referred to as interference effects. These interference effects manifest themselves as non-vertical resist profiles, line width variations and reflective notching.
Some materials and methods have been proposed for reducing the amount of light that is reflected off substrates and/or underlying circuit elements during resist patterning. One method alters the exposure tool so that the photo-resist is radiated with multiple wavelengths. This method provides some benefit by making the process less sensitive to the constructive or destructive interference of one particular wavelength. However, it does not substantially reduce the amount of light reflected through unexposed photo-resist regions during the exposure step nor are the multiple wavelengths amenable to forming sub-micrometer design features. Furthermore, multiple-wavelength exposures place a great demand for color-corrected optics, and are hence not often used.
An alternative method to minimize the deleterious effects of reflected light is to increase the absorption coefficient of the photo-resist. Essentially, the photo-resist is "dyed" so that it absorbs the wavelength of light used during the exposure step. In this way, the reflected light can be prevented from traveling an extended distance through the photo-resist. However, the dye attenuates the incident light as well as the reflected light. Therefore, the intensity of incident radiation diminishes as it travels through the photo-resist. As such, these dyes necessitate much higher exposure doses than the typical transparent photo-resist. Furthermore, use of these dyed resists can result in underexposure of the bottom region of the photo-resist material and overexposure of the top region. This can again result in unacceptable resist profiles.
To overcome these problems, an alternative method has been suggested involving the application of a first coating of a reflective suppression material on top of the substrate before the photo-resist is applied. This coating reduces the amount of light reflected from the underlying substrate during the photo-resist step. In this way the amount of reflected light is reduced without causing the actinic radiation to be attenuated as it travels through the photo-resist. Exemplary anti-reflective coatings of this type include inorganic materials such as Cr.sub.2 O.sub.3, or TiW. Although these materials are effective at reducing the amount of light reflected from the substrate topography, they are difficult to apply uniformly onto the substrate and difficult to remove after the exposure step. As such, these materials often remain and become incorporated into the device. Since the nature of these materials can be incompatible with the final device, these materials are often unsuitable for general use as anti-reflective coatings.
Anti-reflective coatings formed from organic materials also have been suggested for use in reducing the deleterious effects of reflective light. These organic materials can also be spun onto the surface of the substrate prior to application of the photo-resist material. These materials can include a polymer dye that is absorptive either at a specific wavelength or at a broad range of wavelengths. As in the case of the dyed photo-resist, these coatings work by attenuating the actinic radiation as it passes through the anti-reflective coating. Although these materials are effective at absorbing light over a wide range of wavelengths, they have been found to be largely ineffective at shorter wavelengths of light such as 193 nm. Therefore, these materials are less useful at the wavelengths that produce sub-micrometer design features.
Furthermore, many of these anti-reflective coatings can diffuse into photo-resist materials. These coatings that migrate into the photo-resist material can poison the resist chemistry that occurs during the exposure step. As such, these materials have a deleterious impact on line widths and resist profiles.
At least one research group has proposed anti-reflective coatings for use at 193 nm. In an article entitled, "An Anti-Reflection Coating for Use With PMMA at 193 Nanometers", Yen et al. J. Electrochem. Soc., Vol. 139, No. 2, February 1992, at pages 616-619, researchers suggest an anti-reflective coating can be developed that is specifically tailored for the 193 nanometer wavelength. However, the materials disclosed within this paper, must be photoactivated in a separate step by radiating the coated substrate with deep UV (260 nanometer) radiation and subsequent annealing. Moreover, it appears that the disclosed anti-reflection coating is adapted for use only with PMA photo-resists. Although the PMMA is a commonly used photo-resist material it is not considered a candidate material for volume production of sub-0.25-micrometer features.
As the properties of photo-resists differ considerably from one type of photo-resist to another, there exists a need for more compatible and more suitable materials and methods for reducing the amount of reflected light that occurs during the photo-lithographic process. In particular, an anti-reflective coating for use at 193 nanometers that is optically compatible with the more complex, multi-component photo-resist materials, and which is substantially insoluble within these photo-resist materials, would overcome a major obstacle to the practicality of 193 nanometer photo-lithography.